Introduction
Osteochondral defects of the talar dome are mostly caused by a traumatic event. They may lead to deep ankle pain on weight bearing, prolonged swelling, diminished range of motion, and synovitis. The mean cartilage thickness of the talar dome is in the range 1.2 to 1.4 mm. Cartilage defects are strongly linked to the subchondral bone. The main target in the treatment of talar osseochondral defects (OCDs) is repair of the subchondral bone. A healthy restored subchondral bone plate would decrease the pain, improve the load-bearing capacity of the ankle, and improve chondrocyte survival in the remaining cartilage
Treatments
Treatment options for OCDs are numerous. The highest success rates were reported for bone marrow stimulation, such as drilling or microfracturing, (85%) and osteochondral autograft transfer (87%). Because osteochondral autograft transfer can cause knee morbidity, the conclusion was that debridement and bone marrow stimulation remains the treatment of choice for primary OCDs (i.e., those without previous surgery) up to 15 mm.
During debridement and bone marrow stimulation, the OCD is preferably approached by anterior ankle arthroscopy with the ankle in full plantar flexion for adequate exposure of the defect.
However, the ankle is a congruent joint with limited surgical access. Some defects are located so far posteriorly that they may not be accessible by anterior ankle arthroscopy.
Lesions after failed previous surgery or large lesions can be treated by various alternative surgical methods, including autologous cancellous bone grafting, osteochondral autograft transfer, and autologous chondrocyte implantation (ACI). Although successful results can be achieved, disadvantages of these secondary methods include pain at the donor site, limited availability of graft material, and two surgical procedures in the case of ACI. An alternative without these disadvantages would be desirable. The methods mentioned above are sometimes combined with osteotomy for better access to the talar dome.
Metal Implants
Because of the disadvantages of current secondary treatment methods, metal resurfacing inlay implants have been developed and started to reach the market. A precise surgical technique is required in terms of implantation depth, position, and orientation because of the biomechanical properties of the ankle joint. A protruding implant may damage the opposite cartilage by causing excessive contact pressures during loading, which is thought to be due to “plowing” of the cartilage. On the other hand, a deep implant might result in collapse of the adjacent cartilage due to insufficient support.
If the lesion is located on the medial side of the talar dome (ca 60% of cases), most of these treatment options require a medial malleolar osteotomy in order to obtain access to the talar dome. A medial malleolar osteotomy is an established approach for the operative treatment of medial osteochondral defects of the talar dome and fractures of the talar body. There are different varieties of osteotomy to get access to the talar dome. A commonly used osteotomy is the oblique medial malleolar osteotomy, which is a crucial step in the surgical implantation of the resurfacing inlay implants. To obtain a congruent joint surface after refixation, the osteotomy should be directed perpendicularly to the articular surface of the tibia at the intersection between the tibial plafond and medial malleolus. At an instructional course on the metallic implantation technique, surgeons experienced technical difficulties performing a successful medial malleolar osteotomy. The difficulties included sawing at an angle that allowed refixation of the distal fragment without creating an articular incongruence, as well as identifying the intersection between the tibial plafond and medial malleolus. Thus, knowledge of the angle of the osteotomy relative to an anatomic landmark such as the long tibial axis would be helpful for use during surgery, as well as surgical tricks to identify the intersection.
It has been reported that a medial malleolar osteotomy frequently led to local osteoarthritis and less favorable clinical findings than arthrotomy without osteotomy.
The advantages of implants over complete replacement of the joint have stimulated a further development of smaller implants that can be implanted with less invasive surgery. In this development there has also been an effort to achieve small joint implants, suitable for repair of a small bone and/or cartilage injury that have a minimal influence on the surrounding parts of the joint. In the current development, such small implants are designed with an implant body that may be formed as a mushroom cap with a hard surface to face the articulating side of the joint and a bone contacting surface engaging the bone below the damaged part of cartilage. The shape and the curvature of the articulating surface of the implant may be designed to be a reconstitution of the shape and the curvature of the part of the joint when it was undamaged. Such implants are usually designed as mushrooms with an implant body or head and with a peg or a rod projecting from the bone contacting side of the implant body for anchoring the implant into the bone. The cap of the mushroom for repair of the talus dome is often slanted or irregular to conform to the shape of the original undamaged ridge or dome of the talus.
There is a lack of talar metal resurfacing implants with an exact match of the undamaged talar anatomy, as well as metal resurfacing implants that can be placed not only at the edge of the medial talar dome, but placed more centrally on the medial talar dome.